A battery such as a Lithium-Ion (Li-Ion) battery is formed by rechargeable battery cells. To increase the voltage capability, the cells are connected in series thereby forming a pack with each unit of the pack consisting of a cell. To increase the current capability, the cells are connected in parallel such that each unit of the pack consists of two or more cells connected in parallel.
As the battery pack is charged and discharged as a single unit, differences in the cells in capacities, temperatures, internal chemical characteristics, internal resistance, degradation, and the like can cause cell imbalances in the form of charge variations. Imbalanced cells can cause over-charging and over-discharging damage and thereby decrease the storage capacity and lifetime of the battery pack. In particular, a cell which experiences deviant behavior is a candidate to overvoltage during charging events. Thus, cell balancing is necessary.
Two groups of cell balancing methods include passive balancing and active balancing. In passive balancing, extra charge of the high charged cells (e.g., the cells having a charge greater than the average charge of all of the cells) is dissipated into a shunt resistor. This leads to energy inefficiency especially for applications requiring relatively high electric energy. In active balancing, extra charge of the high charged cells is removed and then transferred to the low charged cells (e.g., the cells having a charge lower than the average charge of all of the cells).
Active cell balancing methods can be grouped into two types: charge shuttling and energy converting. An example of a charge shuttling active balancing method is the charge shuttle (flying capacitor) charge distribution method. In this method, a capacitor is switched sequentially across each cell in the series chain. The capacitor averages the charge level on the cells by picking up extra charge from the high charged cells and then dumping the extra charge into the low charged cells. This process can be speeded up by programming the capacitor to repeatedly transfer extra charge from the highest charged cell (e.g., the cell having the highest charge) to the lowest charged cell (e.g., the cell having the lowest charge). Efficiency is reduced as the cell voltage differences are reduced. This method is also fairly complex and requires relatively expensive electronics.
An example of an energy converting active balancing method is the flyback transformer charge distribution method. In this method, the primary winding of a transformer is connected across the battery pack (i.e., the primary winding is connected to both ends of the battery pack). Pursuant to a “switched transformer” arrangement, a secondary winding of the transformer is configured to be switched across the individual cells. Alternatively, pursuant to a “shared transformed” arrangement, each cell has its own secondary winding which can be switched into the primary winding. In either arrangement, current is taken from the entire pack and is switched into the transformer. In turn, the transformer output is delivered to the low charged cells. As such, this method is used to take pulses of energy as required from the full battery pack, rather than small charge differences from a single cell, to top up the remaining cells. This method averages the charge level as with the charge shuttle flying capacitor charge distribution method, but avoids the problem of small voltage differences in the cells and is consequently much faster. Each secondary winding has to be well-balanced or else the secondary windings will contribute to the balancing problem.
Taking into account cell equalization times, it is desirable to implement methods for fast cell balancing to decrease the total charging time and increase energy efficiency. It is further desired that such methods avoid unnecessary switching during equalization as such switching can degrade efficiency of balancing, reduce switch life, and increase high-frequency noise.